Gas analyzer cuvettes

ABSTRACT

Gas analyzer systems which include: (1) a transducer for outputting a signal indicative of the concentration of a specified gas in a sample which may contain that gas, and (2) an airway adapter or cuvette with a flow passage for confining the sample to a particular path traversing the transducer. The cuvettes feature radiant energy transmitting windows which are flush mounted in apertures on opposite sides of the cuvette flow passage and are fabricated from a polymer such as biaxially oriented polypropylene which is malleable, yet resistant to wrinkling, warping, and other forms of distortion. Retainer rings keep the windows flat and distortion free with an accurately reproducible spacing between the windows.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of application No. 07/598,984filed Oct. 17, 1990, now U.S. Pat. No. 5,369,277. The parent applicationis a continuation-in-part of application No. 07/528,059 filed May 23,1990, now abandoned.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to novel, improved cuvettes; to methodsfor manufacturing those devices; and to novel, improved gas analyzersemploying the cuvettes.

Definitions

Cuvette: a device which is configured to contain a static or dynamic gassample and in which the concentration of a designated gas in the samplecan be ascertained.

Sampling Passage: a cavity in a cuvette which confines a sample composedof one or more gases to a particular flow path traversed by an opticalflow path between an infrared radiation emitter and an infraredradiation detector (dynamic sample) or to a particular location along aflow path of that character (static sample).

BACKGROUND OF THE INVENTION

U.S. Pat. Nos. 4,859,858 and 4,859,859 were issued to Knodle et al. on22 August 1989; and U.S. Pat. No. 5,153,436 was issued to Apperson etal. on 6 October 1992. These three patents disclose analyzers foroutputting a signal indicative of the concentration of a designated gasin a sample being monitored by the apparatus.

The gas analyzers disclosed in the '858, '859, and '436 patents are ofthe non-dispersive type. They operate on the premise that theconcentration of a designated gas can be measured by: (1) passing a beamof infrared radiation through the gas, and (2) then ascertaining theattenuated level of the energy in a narrow band absorbable by thedesignated gas. This is done with a detector capable of generating aconcentration proportional electrical output signal.

One important application of the invention at the present time ismonitoring the level of carbon dioxide in the breath of a medicalpatient. This is typically done during a surgical procedure as anindication to the anesthesiologist of the patient's condition, forexample. As the patient's wellbeing, and even his life, is at stake, itis of paramount importance that the carbon dioxide concentration bemeasured with great accuracy.

In a typical instrument or system employing non-dispersive infraredradiation to measure gas concentration, including those disclosed in the'858, '859, and '436 patents, the infrared radiation is emitted from asource and focused into a beam by a mirror. The beam is propagatedthrough a sample of the gases being analyzed. After passing through thebody of gases, the beam of infrared radiation passes through a filter.That filter reflects all of the radiation except for that in a narrowband centered on a frequency which is absorbed by the gas of concern.This narrow band of radiation is transmitted to a detector whichproduces an electrical output signal proportional in magnitude to themagnitude of the infrared radiation impinging upon it. Thus, theradiation in the band passed by the filter is attenuated to an extentwhich is proportional to the concentration of the designated gas. Thestrength of the signal generated by the detector is consequentlyinversely proportional to the concentration of the designated gas andcan be inverted to provide a signal indicative of that concentration.

In a typical medical application of the gas analyzers just described, acuvette is employed to sample a patient's gas exchange via a nasalcannula or to connect an endotracheal tube to the plumbing of amechanical ventilator. The cuvette confines expired and inspired gasesto a specific flow path; and it furnishes an optical path between aninfrared radiation emitter and an infrared radiation detector unit, bothcomponents of a transducer which can be detachably coupled to thecuvette.

A typical cuvette is molded from an appropriate polymer, and it has apassage defining the flow path for the gases being monitored. Theoptical path traverses the flow path with apertures in the wall of thecuvette and aligned along and on opposite sides of the flow passageallowing the beam of infrared radiation to enter the cuvette; traversethe gases in the flow passage; and, after being attenuated, exit fromthe cuvette to the filter and radiation detector. Transmissive sapphirewindows in the apertures confine the gases to the cuvette flow passageand keep out foreign matter while minimizing the loss of infrared energyas the beam enters and exits from the cuvette.

Sapphire is a relatively expensive material. Consequently, cuvettes ofthe character just described are invariably cleaned, sterilized, andreused. The cleaning and sterilization of a cuvette is time-consumingand inconvenient; and the reuse of a cuvette may be perceived as posinga significant risk, especially if the cuvette was previously employed inmonitoring a patient suffering from an infectious disease. Anotherdisadvantage of using sapphire windows is that adhesive bonding is theonly viable technique for mounting the windows to the cuvette. Thistechnique is slow and expensive, and care must be taken that the windowsare accurately positioned.

Efforts have been made to reduce the cost of cuvettes by replacing thesapphire cuvette windows with windows fabricated from a variety ofpolymers. These efforts have heretofore been unsuccessful.

One, if not the major, problem encountered in replacing sapphire cuvettewindows with windows fabricated from a polymer is that of establishingand maintaining a precise optical path length through the sample beinganalyzed. This is attributable to such factors as a lack of dimensionalstability in the polymeric material, the inability to eliminatewrinkles, and the lack of a system for retaining the windows at preciselocations along the optical path.

One proposal for solving this problem is made in U.S. Pat. No. 5,067,492issued 26 November 1991 to Yelderman et al. The patented approach is tosqueeze the cuvette between two housing segments of the transducer withwhich it is used in the course of assembling the cuvette to thetransducer. If the same transducer is employed and if its housing isdimensionally stable, this will in theory ensure that the distancebetween the two cuvette windows is the same each time the same cuvetteis used.

This solution has major drawbacks. Squeezing the cuvette is apt towrinkle or otherwise distort the perhaps initially not distortion-freeplastic windows; and this may affect the transmittance of the windowsenough to cause a significant error in the concentration of the gasbeing monitored. Furthermore, in the Yelderman et al. design, thewindows are spaced inwardly from the flow passage-associated ends of theoptical path apertures. This leaves cavities communicating with the flowpassage in which unwanted debris can collect; and these crevices canadversely affect the flow of gases through the cuvette. Also, thestructure employed to position and retain the plastic windows in thebody of the cuvette is important; and Yelderman et al. contains only thesketchiest of suggestions of how this might be accomplished, let alone adescription of a window retaining system that would minimize wrinklesand other distortions and accurately hold the windows in place,particularly considering the squeezing of the cuvette needed to assemblethe cuvette to its adapter. Another problem with the Yelderman et al.hardware is that of assembling the cuvette to the adapter because of theinterference fit between these two components.

SUMMARY OF THE INVENTION

Now invented and disclosed herein are new and novel cuvettes which canbe manufactured cheap enough that it is practical to dispose of themafter use with a single patient or if the cuvette becomes unusable dueto contamination or a dirty window, for example. At the same time, thosenovel cuvettes are free of the defects and drawbacks of previouslyproposed cuvettes with sapphire-substitute windows including the onedisclosed in the above-cited Yelderman et al. patent.

The cuvettes disclosed herein resemble those heretofore proposed to theextent that they include a flow passage for the gas(es) being monitored,apertures to and from the flow passage for transferring infraredradiation propagated along an optical path traversing the gases in theflow passage, and radiation transmitting windows in those apertures forconfining the gas(es) to the flow passage. However, the cuvettes of thepresent invention differ from those heretofore proposed in a number ofimportant respects.

One is the material from which the windows are fabricated. Theconventional polyethylenes and polypropylenes heretofore proposed byYelderman et al. and others is inherently not very strong in thethicknesses required for acceptable infrared transmission. As a resultof this lack of strength, such windows will stretch and move whenexposed to the changes in airway pressure inherent in breathingcircuits. The resulting changes in pathlength naturally cause variationsin system calibration and accuracy. Additionally, such windows are verysusceptible to damage in the course of normal handling and installationin the breathing circuit.

Instead of the conventional polymers employed by Yelderman et al., thewindows of the novel cuvettes disclosed herein are preferably fabricatedfrom a malleable homopolymer, most preferably a biaxially orientedpolypropylene (BOPP), in the thickness range of 0.001 in to 0.005 in.Polymers of this character are widely available, strong in thin gauges,malleable, and relatively transparent to infrared radiation; and muchbetter control over the thickness of the film can be obtained.

BOPP offers numerous advantages. The high strength of BOPP allows thewindows to be self-supporting and sufficiently rigid to eliminate theneed to secondarily define the pathlength with the sensor in order toprevent movement with changes in airway pressure (In the preferredembodiment described in Yelderman et al., "portions of the gas analyzerhousing protrude and slightly squeeze the optical windows of the adapterbody so as to accurately locate the optical windows . . . so that themembranes of the optical windows are a predetermined distance from eachother").

BOPP is sufficiently strong to maintain, without relaxation, the tensionimparted during assembly to produce a flat, distortion-free window.Also, BOPP windows are more durable and less apt to be damaged andrendered unusable in normal use. BOPP films are sufficiently strong towithstand the stresses of the mechanical installation and retention. Bythe nature of the manufacturing process for BOPP films, thicknessvariations are minimized, thus providing improved reproducibility of thepathlength and less variability in the optical properties.

Because BOPP's are inexpensive, it is practical to dispose of cuvetteswith windows fabricated from these materials after a single use or ifthe cuvette should become dirty, contaminated, or otherwise unusablewithout cleaning, disinfection, and the like.

A zero calibration can be performed on the transducer after the cuvetteis assembled to it. This calibration is important because it allows oneto achieve optimum accuracy by zeroing out spectral tolerances in thewindow material. This eliminates errors which might otherwise be causedby deviations from nominal tolerances in the lot-to-lot chemicalcomposition as well as thickness and other physical specifications ofthe stock material.

Another advantage of the present invention is that the polymericmaterials from which the windows are formed are also, by virtue of theirbiaxial polymer chain orientation, resistant to wrinkling, warping, andother forms of accuracy-affecting distortion. Yet they are malleable,which makes the material easy to form to a shape in which they span andseal the apertures in the cuvette.

Yet another advantage of employing the preferred polymeric windowmaterials is that cuvettes with windows formed from those materials arebackwards compatible. That is, cuvettes with windows fabricated fromsuch materials can be substituted for cuvettes with sapphire windowswithout redesigning the transducers with which the cuvettes are used.

Another significant feature of the present invention is the mounting ofthe windows with their inner faces flush with that surface of thecuvette bounding the flow passage. This eliminates debris trapping andflow affecting nooks and crannies such as those in the Yelderman et al.cuvettes.

Still another important feature of the present invention is the use ofskeletal snap-in retainers or rings to immobilize the windows at preciselocations in the cuvette bodies. These retainers can also be employed tocollapse the window-forming material around those components as they areinstalled. This provides gastight seals between the retainers and thebody of the cuvette. At the same type, this novel technique forinstalling and sealing the cuvette windows eliminates the need to heatthe windows if heat sealing were employed. This is important as heatcould well ruin the optical flatness required for accurate carbondioxide measurement.

Other advantages attributable to the snap ring system employed to holdthe windows in place are:

(1) fast, reliable, inexpensive manufacture of the adapters is promoted;

(2) adhesives and the attendant requirements for careful handling,dispensing, and curing are eliminated;

(3) heat sealing is not required;

(4) the adapter body can be molded from high strength polymers whichhave better dimensional stability than the conventional polyethylenesand polypropylenes employed by Yelderman et al. as the adapter body andthe windows do not have to be fabricated from similar material.

The apertures in the cuvettes are configured to precisely locate thewindows along the gas traversing optical path; and the cuvette isfabricated from a rigid, deformation resistant material. As a result thelength of the optical path can be depended upon to remain constantwithin the very close tolerances required for accurate measurementwithout employing the Yelderman et al. cuvette squeezing assemblytechnique or another equally inconvenient method of providing anaccurate optical path length.

The cuvette apertures are also configured to provide the just-discussedsnap-in assembly of the window retainers to the body of the cuvette andeliminate the need for heat sealing or adhesives.

The inner, flow passage-associated ends of the cuvettes are preferablyso dimensioned that a window or retainer can not pass through theaperture if it is accidentally dislodged. This is a significant safetyfeature as it keeps a dislodged retainer or window from perhaps passingto a mechanical ventilator or other equipment or, even worse, beingpumped through an endotracheal tube into a patient's lungs.

The objects, advantages, and features of the present invention will beapparent to the reader from the foregoing and the appended claims and asthe ensuing detailed description and discussion proceeds in conjunctionwith the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded view of a gas analyzer comprising: (a) an airwayadapter type of cuvette which embodies the principles of the presentinvention and which provides a particularized flow path for a gas beinganalyzed, and (b) a transducer which generates a beam of infraredradiation, propagates that beam along an optical path traversing thecuvette flow path, detects the beam as attenuated by a designated gas inthe flow path, and outputs a signal indicative of the concentration ofthe designated gas;

FIG. 2 is a section through the airway adapter/transducer assembly;

FIG. 3 is an exploded view comprised of: a transverse section throughthe airway adapter; one of two windows which transmit infrared radiationto the airway adapter flow passage and, after it has been attenuated bythe designated gas, from the flow passage to the exterior of the airwayadapter; and a retainer which holds the window in place in the body ofthe airway adapter;

FIG. 4 is a simplified view of one machine that can be used to installthe windows and retainers in the airway adapter of FIG. 1 and to sealthe gaps between the retainers and the apertures in which they areinstalled;

FIG. 5 is a fragmentary section through the airway adapter showing oneof its radiation transmitting windows in an initial stage of beinginstalled with the technique depicted in FIG. 3 and the machineillustrated in FIG. 4;

FIG. 6 is a view like FIG. 5 but showing the window completelyinstalled;

FIG. 7 is a simplified vertical section through a second gas analyzerwith a cuvette which also embodies the principle of the presentinvention;

FIG. 8 is a top view of the cuvette of FIG. 7;

FIG. 9 is a view like FIG. 7 of a third gas analyzer with a cuvettewhich embodies the principles of the present invention;

FIG. 10 is a top view of the cuvette shown in FIG. 9;

FIG. 11 is a fragment of FIG. 4 to an enlarged scale;

FIG. 12 is a perspective view of a sampling airway adapter employing theprinciples of the present invention;

FIG. 13 is a second perspective view of the sampling airway adapter; and

FIG. 14 is a section through FIG. 12 taken substantially along line14--14 of the latter figure.

DETAILED DESCRIPTION OF THE INVENTION

Referring now the drawing, FIGS. 1 and 2 depict a gas analyzer 20composed of: (1) an airway adapter 22 embodying the principles of thepresent invention and designed for connection between an endotrachealtube inserted in a patient's trachea and the plumbing of a mechanicalventilator; and (2) a complementary transducer 24 for outputting: (a) asignal proportional in magnitude to the concentration of carbon dioxideflowing through airway adapter 20, and (b) a reference signal. Thesesignals can be ratioed in the manner disclosed in the above-cited '858,'859, and '436 patents to provide a third signal, accurately anddynamically representing the concentration of the carbon dioxide flowingthrough the airway adapter.

FIG. 1 shows primarily the polymeric housing 26 of transducer 24. Thetransducer also includes an infrared radiation emitter unit or source 28and a detector unit 30 (see FIG. 2).

That casing 26 of transducer 24 in which the infrared radiation emitterunit 28 and detector unit 30 are housed has first and second endsections 32 and 34 with a rectangularly configured gap 36 therebetween.With the transducer assembled to airway adapter 22, the two sections 32and 34 of transducer casing 26 embrace airway adapter 22, integratingthe adapter and transducer into a single, easily handled unit orassembly.

Optically transparent windows 38 and 40 are installed in transverselyaligned apertures 42 and 44 provided in the inner end walls 46 and 48 oftransducer housing 26. These windows allow the beam 49 of infraredradiation generated in unit 28 in the left-hand end section 32 oftransducer housing 26 to pass along optical path 50 to airway adapter 22and from the airway adapter to the detector unit 30 in the right-handsection 34 of the transducer housing. At the same time, windows 38 and40 keep gases from escaping and keep foreign material from penetratingto the interior of the transducer casing.

Unit 28 is employed to emit infrared radiation, to form that energy intobeam 49, and to propagate the beam along the optical path 50 traversingthe gas being monitored as it flows through airway adapter 22. Unit 20includes: (1) a thick film infrared radiation emitter 52, (2) an emitterprotecting cap 54, (3) a parabolic, gold-over-copper-plated mirror 56for collating the energy outputted from emitter 52, and (4) an emitter-,mirror-, and cap-supporting base 58.

Detector unit 30 includes a boxlike housing 60 mounted on a printedcircuit board 62. A monolithic, heat conductive, isothermal support 64is installed in housing 60. Supported from and mounted in support 64are: (1) data and reference detectors 66 and 68, (2) a beam splitter 70,(3) detector heaters 72 and 74, and (4) a thermistor-type currentflow-limiting device 76. The system in which heaters 72 and 74 andthermistor device 76 are incorporated (see above-cited U.S. Pat. No.5,153,436) is employed to keep the reference and data detectors 68 and66 at the same, selected temperature, typically with a tolerance of notmore than 0.01° C.

Detectors 66 and 68 are preferably fabricated with lead selenidedetector elements because of the sensitivity which that materialpossesses to electromagnetic energy having wavelengths which are apt tobe of interest.

Each of the two detectors 66 and 68 is mounted in a stepped recess 78opening onto the front side of heat conductive support 64. A gap 80around the periphery of the detector and between the detector andisothermal support 64 electrically isolates the detector from the alsoconductive, isothermal support.

Beam splitter 70 has a generally parallel-epipedal configuration. Thiscomponent is fabricated from a material such as silicon or sapphirewhich is essentially transparent to electromagnetic energy withwavelengths of interest. The exposed front surface 82 of the beamsplitter is covered with a coating 84 which will reflect to datadetector 66 as indicated by arrow 86 in FIG. 2 energy having awavelength longer than about 4 microns. The energy of shorterwavelengths is, instead, transmitted through the beam splitter toreference detector 68 as is suggested by arrow 88 in the same figure.

Bandpass filters 90 and 92 limit the electromagnetic energy respectivelyreflected from and transmitted by beam splitter 70 and impinging upondetectors 66 and 68 to energy in selected bandwidths. Reference detectorfilter 92 in detector unit 30 is centered on a wavelength of 3.681 μmand has a half power bandwidth of 0.190 μm. The data detector bandpassfilter 90 is centered on a wavelength of 4.260 μm and has a bandwidth of0.10 μm. This is two times narrower than the band passed by filter 92.The carbon dioxide absorption curve is fairly narrow and strong, andbandpass filter 90 centers the transmission band within that absorptioncurve. Therefore, if there is a change in carbon dioxide level in thegas(es) being analyzed, the maximum modulation for a given change incarbon dioxide level is obtained.

Each of the bandpass filters 90 and 92 is installed in that steppedrecess 78 in monolithic, isothermal support 64 in which the associateddetector 66 or 68 is mounted.

The upper edge 94 of beam splitter 70 is fitted into a recess 96 inmonolithic, isothermal support 64 midway between the bandpass filter 90in front of data detector 66 and the bandpass filter 92 in front ofreference detector 68. The opposite, lower part 98 of the beam splitteris fixed to an inclined, integral lip 100 which extends inwardly fromdetector unit casing 60.

The electromagnetic energy in beam 49 reaches beam splitter 70 throughan aperture 102 in the front side of detector unit casing 60. A sapphirewindow 104 spans aperture 102 and keeps foreign material frompenetrating to the interior of housing 60.

To exclude extraneous energy, and thereby ensure that only the energy inbeam 49 reaches beam splitter 70, light traps 106 and 108 are provided.The first of these is a triangularly sectioned, inwardly extendingprojection of monolithic isothermal support 64. The second, cooperatinglight trap 108 is aligned with, fixed in any convenient fashion to, andextends inwardly from the casing-associated ledge 100 supporting beamsplitter 70.

The operation of transducer 24 is believed to be apparent from thedrawings and the foregoing, detailed description of the transducer.Briefly, however, electromagnetic energy in the infrared portion of thespectrum is generated by heating the source or emitter 52 of emitterunit 28, preferably by applying bipolar pulses of electrical energy tothe emitter unit. The energy thus emitted is propagated toward theconcave, emitter unit mirror 56 as shown by arrow 110 in FIG. 2. Mirror56 collimates and focuses this energy and propagates it in the form ofbeam 49 along optical path 50 across the gas(es) flowing through airwayadapter 22.

Energy in a species specific band is absorbed by the gas of interestflowing through the airway adapter (typically carbon dioxide) to anextent proportional to the concentration of that gas. Thereafter, theattenuated beam passes through the aperture 102 in detector unit casing60, is intercepted by beam splitter 70, and is either reflected towarddata detector 66 or transmitted to reference detector 68. The bandpassfilters 90 and 92 in front of those detectors limit the energy reachingthem to specified (and different) bands. Each of the detectors 66 and 68thereupon outputs an electrical signal proportional in magnitude to theintensity of the energy striking that detector. These signals areamplified and then ratioed to generate a third signal accuratelyreflecting the concentration of the gas being monitored. The signalprocessor used for this purpose is independent of airway adapter 22 andtransducer 24, is not part of the present invention, and willaccordingly not be disclosed herein.

Resistance heaters 72 and 74 and thermistor 76 are installed inisothermal support 64, producing efficient, conductive heat transferbetween the heaters and the support. The spatial relationship betweenheater 72 and data detector 66 and between heater 74 and referencedetector 68 are identical, and the spatial relationship betweenthermistor 76 and each of the heaters 72 and 74 is also identical.Furthermore, the two heaters 72 and 74 are so located with respect tothe associated detectors 66 and 68 that the thermal energy emitted fromthe heaters travels first across the detectors and then across thecurrent flow-limiting thermistor 76 to heat dumps provided by gaps 118and 120 between isothermal support 64 and detector unit housing 60 (Theheat flow paths are identified by arrows 122 and 124 in FIG. 2). As aconsequence of the foregoing and the high thermal conductivity ofisothermal support 64, the data and reference detectors 66 and 68 canreadily be maintained at the same temperature as is required foraccuracy of gas concentration measurement.

Referring more specifically to FIGS. 1-3, airway adapter 22 embodies theprinciples of the present invention and is typically molded from apolycarbonate or a comparable rigid, dimensionally stable polymer. Theairway adapter has a generally parallelepipedal center section 126 andtwo cylindrical end sections 128 and 130 with a sampling (or flow)passage 132 extending from end-to-end through the adapter. Left-hand andright-hand airway adapter end sections 128 and 130 are axially alignedwith center section 126.

The central section 126 of airway adapter 22 provides a seat fortransducer 24. An integral, U-shaped casing element 134 positivelylocates transducer 24 endwise of the adapter and, also, in thattransverse direction indicated by arrow 136 in FIG. 1. That arrow alsoshows the direction in which airway adapter 22 is displaced todetachably assemble it to transducer 24. The airway adapter snaps intoplace (see the above-cited '858 and '859 patents); no tools are neededto assemble or remove the adapter.

Apertures 138 and 140 are formed in the center section 126 of airwayadapter 22. With transducer 24 assembled to the airway adapter, theseapertures are aligned along optical path 50. Thus, infrared radiationbeam 49 can travel from the infrared radiation emitter unit 28 intransducer 24 transversely through airway adapter 22 and the gas(es)flowing through airway adapter flow passage 132 to the infraredradiation detector unit 30 of transducer 24.

To: (a) keep the gases flowing through airway adapter sampling passage132 from escaping through apertures 138 and 140 without attenuating theinfrared radiation traversing optical path 50, and (b) keep foreignmaterial from the interior of the airway adapter, the apertures 138 and140 are sealed by windows 142 and 144 which have a high transmittancefor radiation in the infrared portion of the electromagnetic spectrum.

As discussed above, that casing 26 of transducer 24 in which the sourceunit 28 and detector unit 30 are housed has first and second endsections 32 and 34 with a rectangularly configured gap 36 therebetween.With the transducer assembled to airway adapter 22, the two sections 32and 34 of transducer casing 26 embrace those two inner side walls 146and 148 of airway adapter central section 126 in which energytransmitting windows 142 and 144 are installed. This securely attachesairway adapter 22 to transducer 24 with airway adapter windows 142 and144 and transducer windows 38 and 40 all aligned along optical pathway50, allowing infrared radiation beam 49 to travel from emitter unit 28to detector unit 30 through the gas(es) in sampling passage 132.

Heretofore, the windows of cuvettes such as that shown in FIGS. 1-3 aswell as cuvettes of other configurations have been fabricated fromsapphire because of that material's favorable optical properties;stability; and resistance to breakage, scratching, and other forms ofdamage. However, as discussed above, sapphire windows are expensive; andthis makes it impractical to discard the cuvette after it is used tomonitor a single patient. Instead, the cuvette must be cleaned,sterilized, and reused, which is inconvenient and often perceptuallyhazardous.

It has now been found that the cost of manufacturing a cuvette can bereduced--even to the point of making it practical to dispose of thecuvette after a single use--by fabricating the cuvette windows from anappropriate polymer rather than the many times more expensive,heretofore employed sapphire.

It is essential to accuracy that the polymer transmit a usable part ofthe infrared radiation impinging upon it. As discussed above, thepreferred window material is biaxially oriented polypropylene.

Referring now specifically to FIGS. 3, 5, and 6, windows 142 and 144 areinstalled in airway adapter apertures 138 and 140 such that the innersurfaces 150 and 152 of the windows are flush with the inner surfaces154 and 156 of airway adapter center section side walls 146 and 148.This eliminates any recesses in optical path apertures 138 and 140 inwhich debris might collect and obscure the optical path. Theflush-mounting also minimizes flow resistance and assures that there areno edges which would interfere with the delivery of medication or theintroduction of catheters or other cannula through the airway adapter.

The perimeters 162 and 164 of the two window-receiving apertures 138 and140 have a configuration which is designed to position the energytransmitting window 142 or 144 in its aperture 138 or 140 with thewindow in the above-discussed flush relationship with the inner surface(154 or 156) of the airway adapter side wall in which the aperture isformed. Aperture 140 is typical. The perimeter 164 of aperture 140,which is shown to an enlarged scale in FIGS. 5 and 6, has an innersegment 166. That segment tapers inwardly (toward the centerline 168 ofaperture) from: (1) a plane 170 lying between and parallel to the innerand outer surfaces 156 and 172 of airway adapter side wall 160, to (2)the inner side wall surface 156. A second, integral segment 174 of theaperture tapers inwardly (also toward aperture centerline 168) over thatspan lying between plane 170 and outer side wall surface 172.

Windows 142 and 144 are held in place (or immobilized) in the associatedapertures 138 and 140 in the flush-mounted relationship just describedwith skeletal, elastically compressible, snap-in retainer rings 176 and178, which are typically made from brass. Brass has the advantage thatit is cheap, easily machined, and readily plated with other metals suchas nickel, if desired. However, other materials may be employed in placeof brass although many polymers are not suitable as they would not beable to withstand the forces generated during the installation of theretainers.

The two retainers 176 and 178 are identical; accordingly, only retainer178, best shown in FIGS. 5 and 6, will be described in detail. Thatairway adapter component has a circular configuration. Its periphery hasa first, inner segment 180 which complements the inner segment 166 ofwindow aperture perimeter 164; like the latter, it is tapered towardaperture centerline 168 from plane 170 to airway adapter inner side wallsurface 156. The ring also has an integral, outer segment 182complementing the outer segment 174 of the aperture 140 perimeter. Thatretainer ring segment tapers toward aperture centerline 168 from plane170 to airway adapter side wall outer surface 174.

By virtue of the complementary configurations just described, retainerring 176 snaps into aperture 140. It is thereby positively retained inthat exact location shown in FIG. 6 in which it retains window 144 inaperture 140 in the flush relationship to airway adapter inner wallsurface 156 described above.

Referring now most specifically to FIGS. 3, 5, and 6, the initial stepin installing window 144 is to die cut or otherwise form a circularblank 184 from an appropriately thick sheet of the selected infraredradiation transmitting polymer. As shown in FIG. 3, blank 184 andretainer ring 178 are positioned adjacent airway adapter side wall 160in axial alignment with and centered along the centerline 168 ofaperture 140. The blank and retainer ring are then displaced towardairway adapter 22 and into aperture 140 as indicated by arrow 186 inFIG. 3. As blank 184 and retainer ring 178 are forced through aperture140 toward sampling passage 132, the margin 188 of blank184--particularly if the blank is made from a malleable polymer as ispreferred--is molded first around the inner segment 180 of retainer ring178 (see FIG. 5) and then around the outer segment 182 of the retainerring (see FIG. 6). This results in the marginal segment 188 of the blankforming a gastight seal in the gap 190 between the perimeter 164 ofaperture 140 and the periphery of retainer ring 178 when the blank andretainer ring reach the assembled, or final, FIG. 6 position. At thesame time, the rigidity and toughness of the material and the justdescribed installation process result in the window segment 140 of theblank being placed under tension and therefore being flat and free ofdistortions which might produce an error in the signal outputted bytransducer 24.

One machine for forming blank 184 and for so pressing that blank andretainer ring 178 into aperture 140 as to form and immobilize window 144with the retainer ring snapped into place and the gap 190 between it andthe perimeter 164 of aperture 140 sealed is illustrated in FIGS. 4 and11 and identified by reference character 192. That machine includes anairway adapter-supporting mandrel 194, a die 196, and a hollow punch 198supported by a second mandrel 200 for movement in the arrow 186 andopposite directions (the same machine is of course used to form andinstall window 142 and retainer ring 176).

Airway adapter-supporting mandrel 194 is dimensioned and configured tomatch the sampling passage 132 through airway adapter 22. This mandrelis employed to position the airway adapter with aperture centerline 168in axial alignment with the longitudinal centerline 201 of die 196 andpunch 198. The mandrel also backs up the side wall 148 of the airwayadapter as the window forming blank 184 and retainer ring 178 arepressed into the aperture.

Mandrel 200 is mounted to machine framework 203. An integral annularstop 202 and a frame-associated stop 204 immobilize airway adapter 22 inthe illustrated and just described window installation position relativeto punch 196.

Die 196 is a hollow, stationary, cylindrical component mounted alongcenterline 198 and operationally located adjacent the airway adapter 22in which windows are being installed. A slot 205 extending through thewall 206 of the die at right angles to centerline 198 accommodates arectilinearly displaceable strip 207 of window-forming polymer. Anintegral, annular ledge 208 extending inwardly from die wall 206 intolongitudinally extending central bore 209 immediately above slot 205defines one of two cooperating cutting edges 210 for severing blank 184from strip 207. The second cutting edge is formed on punch 198 and isdescribed below.

Mandrel-supported, hollow punch 198 is an elongated tube. It isdimensioned for a free sliding fit in: (1) the lower segment 212 of thelongitudinally extending bore 209 through die 196, and (2) an axiallyaligned bore 214 through a stationary, punch-and-die supportingcomponent 216 of machine 192. The die-associated end segment 218 ofpunch 198 is tapered outwardly to complement the configuration ofretaining ring peripheral outer segment 182. The sharp cutting edge 219at the free end of punch segment 218 cooperates with the cutting edge210 of die 196 to sever blank 184 from strip 207 as punch 198 isdisplaced relative to die 196 in the arrow 186 direction.

Punch-supporting mandrel 200 is fixed in any appropriate fashion totubular punch 198 and is displaced in the arrow 186 and oppositedirections by a mechanism which has not been shown as it is not part ofthe present invention. The die facing end 220 of mandrel 200 liesinwardly from the cutting edge 219 of punch 198. This leaves an openrecess or cavity 221 for retainer 178. The upper edge 222 of theretainer lies below cutting edge 219 and will not interfere with thecutting blank 184 from strip 206.

Mandrel 200 has a vacuum system including a central cavity 224 openingonto the die facing end 220 of the mandrel and an internal vacuum line226 providing fluid communication between cavity 224 and a vacuum pump(not shown). A porous plug 228, fitted into vacuum cavity 224, promotesa uniform negative pressure profile at the die-facing end of cavity 224and, consequently, a similarly uniform negative pressure profile overthe area of the hollow bore 230 through retainer ring 178.

The vacuum system just described and identified in its totality byreference character 232 is employed to hold the window-forming blank 184on and in axial alignment with the retaining ring 178 installed incavity 221 as punch 198 and the retaining ring 178 are displaced furtherin the arrow 186 direction to press the blank and retaining ring intoaperture 140.

Installation machine 192 is shown in FIG. 4 at the beginning of itsoperating cycle with retainer ring 178 having been placed in cavity 221from the open upper end of the bore 209 in die 196 by dropping itthrough that bore, the hole 234 left by punching the preceding blankfrom strip 207, and the lower part 212 of die bore 209, care being takento ensure that the retainer ring is placed in recess 221 in theorientation shown in FIG. 4. Thus installed in recess 221, retainer ring178 is seated on the upper end 220 of mandrel 200 and backed againstdeformation during the installation process. The upper edge 222 of theretainer lies in the recessed, non-interfering relationship with thecutting edge 184 of punch 198. A protruding, annular, hollow boss 238 atthe upper end 220 of die 196 centers retainer ring 178 about axialcenterline 201, precisely aligning the retainer ring with the aperture140 in which it is to be installed.

The assembly of mandrel 200 and punch 198 is then advanced in the arrow186 direction with the cutting edges 210 and 219 of die 196 and punch198 severing blank 184 from strip 206 and vacuum system 232 trapping theblank on the upper end segment 218 of the punch in axial alignment withretainer ring 178.

As punch 198 continues through and beyond die 196 in the arrow 186direction, blank 184 and retainer ring 178 are pressed into aperture140. This tensions and eliminates distortions in the window-formingportion of the blank and begins the seal-forming deformation of blank184 by virtue of peripheral blank segment 188 being trapped against theouter edge segment 174 of aperture boundary 164 (FIG. 5). Continuedmovement of the punch 198 in the arrow 186 direction: (1) completes theformation of the seal 188 in the gap 190 between the boundary ofaperture 140 and the periphery of retainer ring 178 and the eliminationof distortions in the window-forming material, (2) displaces thewindow-forming segment 144 of the blank into the above-discussedflush-mounted relationship with the inner, sampling passage-definingsurface 156 of airway adapter side wall 160; and (3) snaps retainer 178into place to positively retain the window in place (see FIG. 6). Ifblank 184 is fabricated from the preferred material, the seal 188 isformed without wrinkling, creasing, folding, or puckering of thematerial and without that material being sheared by the retainer. Thispromotes gastight sealing of the gap 190 between the retainer 178 andthe aperture 144 in which it is installed.

Finally, punch 198 and mandrel 200 are retracted to the illustratedposition to ready machine 192 for the next installation cycle.

It was pointed out above that there are many cuvettes in whichpolymeric, infrared radiation transmitting windows may be employed toadvantage in accord with the principles of the present invention. One ofthese is discussed above; and it may be manufactured in different sizessuiting it for adult, neonatal, and other applications. A second suchcuvette and the folded path gas analyzer in which it is employed areshown in diagrammatic form in FIGS. 7 and 8 and identified by referencecharacters 240 and 242, respectively.

Cuvette 242 has top and bottom walls 244 and 246 fabricated of a porous,particulate material trapping polyethylene or other material withsufficient structural integrity and dimensional stability to positivelylocate with a precise distance therebetween two radiation transmittingwindows 248 and 250 embodying the principles of the present invention aselucidated above. These windows may be assembled to cuvette walls 244and 246 and the gaps between the windows and the walls sealed withretainer rings of the character described above. To this end, recesses252 and 254 formed in cuvette top and bottom walls 244 and 246 andopening onto the opposite sides 256 and 258 of the cuvette have V-shapedboundaries as discussed above and best shown in FIGS. 5 and 6 whichcause the retainer rings 264 and 266 to snap into place as they arepressed into the apertures.

Cuvette top and bottom walls 244 and 246, windows 248 and 250, and theside walls 268 and 270 of gas analyzer housing 272 cooperate to define asampling passage 274 for the gas being monitored.

cuvette 242 is installed in gas analyzer housing 272 between itsleft-hand and right-hand ends 276 and 278. Mounted in a thus definedright-hand compartment 280 are an infrared radiation emitter unit 281and an infrared radiation detector unit 282 which may be of thecharacter shown in FIG. 2 and discussed above. Mounted in acomplementary left-hand end compartment 284 are infraredradiation-reflecting mirrors 286 and 288.

Infrared radiation generated by emitter unit 281 is formed into a beam49 as in the unit 28 depicted in FIG. 2. Beam 49 is propagated along thefirst leg 289 of an optical path 290 through cuvette window 250, thegas(es) in cuvette sampling passage 274, and cuvette window 248 tomirror 286. The beam of radiation is attenuated as it passes throughcuvette 242 to an extent proportional to the concentration of the gasbeing monitored.

Mirror 286 turns the beam of infrared radiation 90 degrees, causing itto travel along the second leg 292 of optical path 290 to the second ofthe infrared radiation reflecting mirrors 288. That mirror turns thebeam 90 another degrees. This causes the beam to travel along a thirdoptical path leg 294 in a direction opposite to that in which it wasinitially propagated through: cuvette window 248, the gas(es) in cuvettesampling passage 274, and cuvette window 250 to infrared radiationdetector unit 282, the gas again being attenuated to a degreeproportional to the concentration of the gas being monitored.

The twice attenuated beam of infrared radiation is intercepted bydetector unit 282 which consequentially outputs an electrical signalindicative of the concentration of the monitored gas.

Gas analyzer 240 is designed to monitor gases which reach cuvette 242through one of the two ports 296 and 298 in gas analyzer casing sidewalls 268 and 270. These gases enter the sampling passage 274 in cuvette242 and exit from that passage through the porous cuvette top and bottomwalls 246 and 244 as indicated by double-headed arrow 300 in FIG. 7.

This type of gas analyzer can be employed to advantage as one example tomonitor gases in rooms, buildings, and other confined spaces. Gasanalyzer 240 has the advantage of being compact, and it can be producedat lower cost by virtue of its employing polymeric windows rather thanthe customary sapphire for transferring infrared radiation into and outof cuvette 242.

Referring still to the drawings, FIGS. 9 and 10 depict a folded path gasanalyzer 304 which differs from the analyzer 240 just discussedprimarily by the addition of tubular fittings 306 and 308. Thesefittings communicate with ports 296 and 298 in the side walls 268 and270 of gas analyzer housing 272.

Gas analyzer 304 is employed in medical and other applications whererecirculation of the gas(es) introduced into sampling passage 274 isrequired or advantageous and/or where positive circulation as opposed toconvective or diffusive flow of the gas(es) through the sampling chamberis dictated. The compactness and light weight of gas analyzer 304 isparticularly beneficial in in-line applications such as end tidal carbondioxide monitoring because it can be located close to the patient'sface, which is out of the way and promotes accuracy.

Cuvettes of the diffusion/convective and positive flow types justdescribed above and illustrated in FIGS. 7-10 can vary considerably fromthe specific, exemplary configurations shown in the drawings. Forexample, they may be constructed with a porous, gas transmitting wallwhich extends all the way around the radiation transmitting windows.

Yet another cuvette embodying the principles of the present invention isthe sampling airway adapter illustrated in FIGS. 12-14 and identified byreference character 312. The particular adapter depicted in the drawingsis molded from a polysulfone. However, it may equally well bemanufactured from a polycarbonate or a comparable polymer.

Cuvette 312 has a center section 314 and hollow, cylindrical, left-handand right-hand end sections 316 and 318. An integral platform 320 ofcuvette center section 314 provides a seat for a transducer such as thatdepicted in FIG. 1 and identified by reference character 24. Anintegral, U-shaped casing element 322 positively locates the transducerendwise of cuvette 312 and, also in that transverse direction indicatedby arrow 324 in FIG. 13. Arrow 326 in the same figure shows thedirection in which the transducer is displaced to detachably assembly itto cuvette 312. The airway adapter snaps into place (see the above-cited'858 and '859 patents), and no tools are needed to assemble or removethe adapter.

As is best shown in FIG. 14, a transversely oriented, frustoconicallysectioned sampling chamber 328 is formed in the central section 314 ofcuvette 312. The gas being sampled flows in the direction indicated byarrow 330 in FIG. 14 through the hollow bore 332 of the cuvette'sleft-hand end section 316 to the sampling chamber where theconcentration of a particular specie such as carbon dioxide is measured,using the non-dispersive infrared radiation technique discussed above.Thereafter, the sample is discharged from chamber 328 through acommunicating passage 334 in the central cuvette section 314 and thehollow bore 336 of right-hand cuvette section 318 as suggested by arrow338.

Apertures 340 and 342 are formed in, and on opposite sides of, thecenter section 314 of cuvette 312. With a transducer of the typedepicted in FIG. 1 assembled to cuvette 312 these apertures are alignedalong optical path 344. Thus, infrared radiation outputted from theemitter unit in the transducer can travel in the arrow 344 directionthrough sampling passage 328 and the gas or gases therein to thedetector unit of the transducer.

To: (a) keep the gases in sampling chamber 328 from escaping throughapertures 340 and 342 without attenuating the infrared radiationtraversing optical path 344, and (b) keep foreign material from theinterior of cuvette 312, apertures 340 and 342 are sealed by windows 346and 348 which have a high transmittance for radiation in the infraredportion of the electromagnetic spectrum. As in the cuvettes discussedabove, windows 346 and 348 are fabricated from an appropriate polymer,preferably a biaxially oriented polypropylene. Windows 346 may beformed, installed, and retained in place with skeletal retainers 350 and352 of the character discussed above or by any other appropriatetechnique.

The invention may be embodied in many forms without departing from thespirit or essential characteristics of the invention. The presentembodiments are therefore to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims rather than by the foregoingdescription. All changes which come within the meaning and range ofequivalency of the claims are therefore intended to be embraced therein.

What is claimed is:
 1. A gas analyzer for outputting a signal indicativeof the concentration of a designated gas in a gas sample which maycontain that gas, said gas analyzer comprising:a casing which houses aninfrared radiation emitter, an infrared radiation detector, and acuvette for so confining said sample that infrared radiation propagatedalong an optical path between the infrared radiation emitter and theinfrared radiation detector traverses the gas(es) in said sample; saidcuvette comprising wall means and windows fixed in apertures in saidwall means at opposite sides thereof, said wall means and said windowsdefining a sampling passage through which said gas sample can flow alonga straight path extending generally normal to said optical path; saidgas analyzer casing having first and second compartments located onopposite sides of the cuvette; said windows cooperating with the gasanalyzer casing wall means to isolate the first and second compartmentsin the gas analyzer casing from the sampling passage; the infraredradiation emitter and the infrared radiation detector being isolated inthe first compartment with the emitter being so oriented as to directradiation through the cuvette to the second compartment; the gasanalyzer further comprising mirror means isolated in said secondcompartment for redirecting the infrared radiation reaching thatcompartment back through the cuvette to the infrared radiation detector;and the cuvette wall means having sufficient porosity that gas can reachand/or exit from the sampling passage by diffusion or convective flow.2. A gas analyzer as defined in claim 1 which has inlet and outletfittings for introducing a gas into and/or removing it from the samplingpassage.
 3. A gas analyzer as defined in claim 1 in which said windowsare made from a malleable, infrared radiation transmitting polymer.
 4. Agas analyzer as defined in claim 3 in which the cuvette includes:aretainer in each said aperture for keeping the window therein flat anddistortion-free and for immobilizing said windows in a parallel, spacedapart relationship with a selected, accurately reproducible optical pathlength therebetween.
 5. A gas analyzer as defined in claim 4 in whicheach said window is formed from a blank which has a peripheral portionsurrounding the retainer in the same aperture and clamped between theperiphery of the retainer and the boundary of the aperture to seal thegap therebetween.
 6. A gas analyzer as defined in claim 5 in which thepolymer is a biaxially oriented polypropylene.
 7. A gas analyzer asdefined in claim 4 in which those ends of the apertures communicatingwith the sampling passage are sufficiently small to keep the retainersand windows from being displaced into said passage.
 8. A gas analyzer asdefined in claim 1 in which:the cuvette wall means has an inner surfacebounding said sampling passage; and said windows are so located in saidapertures that the inner surfaces of the windows are flush with theinner surface of the wall means.
 9. A gas analyzer for outputting asignal indicative of the concentration of a designated gas in a samplewhich may contain that gas, said gas analyzer comprising:a casing whichhouses an infrared radiation emitter, an infrared radiation detector,and a cuvette for so confining said sample that infrared radiationpropagated along an optical path between the infrared radiation emitterand the infrared radiation detector traverses the gas(es) in saidsample; said cuvette comprising: wall means bounding a sampling passage,apertures in said wall means aligned along the optical path on oppositesides of the sampling passage, and windows in said apertures, theperimeter of each said aperture having an inner segment configured toprovide a retainer stop for locating the window in that aperturerelative to a boundary of the sampling passage and an outer, undercutsegment for holding said retainer against said stop and immobilizingsaid window in the selected location relative to the boundary of thesampling passage; said cuvette dividing the casing into first and secondcompartments located on opposite sides of the cuvette; the infraredradiation emitter and the infrared radiation detector being located inthe first compartment with the emitter being so oriented as to directradiation through the cuvette to the second compartment; and the gasanalyzer further comprising mirror means in said second compartment forredirecting the infrared radiation reaching that compartment backthrough the cuvette to the infrared radiation detector.
 10. Thecombination of a transducer for outputting a signal indicative of theconcentration of a specified gas in a sample which may contain that gasand an airway adapter comprising a housing with means for confining saidsample to a particular path traversing the transducer;said transducercomprising a housing, an infrared radiation emitter, and an infraredradiation detector means; said emitter and said detector means beinglocated in said transducer housing; the airway adapter means forconfining said sample to said particular path being a flow passagethrough the airway adapter housing; and said airway adapter furthercomprising:means for supporting said transducer housing from said airwayadapter with said infrared radiation emitter and said infrared radiationdetector means on opposite sides of the airway adapter flow passage;apertures in said airway adapter housing on opposite sides of the flowpassage which are aligned along an optical path between said infraredradiation emitter and said detector means and thereby so allow infraredradiation to pass from said emitter through said airway adapter and thegas(es) in the flow passage to said detector means that infraredradiation of the wavelengths absorbed by said specified gas isattenuated before it reaches said detector means so that the signalemitted by said detector means reflects the concentration of thespecified gas in said sample; windows fabricated of an infraredradiation transmitting polymer so installed in said apertures as to: (1)keep said gases from escaping through said apertures, and (2) transmitinfrared radiation outputted from said emitter to the flow passage and,after it traverses gas(es) therein, from the flow passage to saidinfrared radiation detector means; and a retainer ring in each saidaperture for keeping the window therein in a flat and distortion-freeconfiguration and for immobilizing said windows in a parallel, spacedapart relationship with a selected, accurately reproducible optical pathlength between; said retainer ring being surrounded by a peripheralsegment of said window; and said retainer ring having an interferencefit with the airway adapter housing around the periphery of saidaperture that provides window stretching pinch points between theretainer ring and the housing and, consequentially, said flat,distortion-free configuration.
 11. The combination of a transducer foroutputting a signal indicative of the concentration of a specified gasin a sample which may contain that gas and an airway adapter with meansfor confining said sample to a particular path traversing thetransducer;said transducer comprising a housing, an infrared radiationemitter, and an infrared radiation detector means; said emitter and saiddetector means being located in said housing; the airway adapter meansfor confining said sample to said particular path being a flow passagethrough the airway adapter; and said airway adapter furthercomprising:means for supporting said transducer housing from said airwayadapter with said infrared radiation emitter and said infrared radiationdetector means on opposite sides of the airway adapter flow passage;apertures on opposite sides of the flow passage which are aligned alongan optical path between said infrared radiation emitter and saiddetector means and thereby so allow infrared radiation to pass from saidemitter through said airway adapter and the gas(es) in the flow passageto said detector means that infrared radiation of the wavelengthsabsorbed by said specified gas is attenuated before it reaches saiddetector means so that the signal emitted by said detector meansreflects the concentration of the specified gas in said sample; andwindows fabricated of a malleable, infrared radiation transmittingpolymer so installed in said apertures as to: (1) keep said gases fromescaping through said apertures, and (2) transmit infrared radiationoutputted from said emitter to the flow passage and, after it traversesgas(es) therein, from the flow passage to said infrared radiationdetector means, the window forming polymer being under tension in alldirections in the plane of the window and consequentially stretched intoa flat, distortion-free configuration; the airway adapter housing havingwall means with an inner surface bounding said flow passage; saidapertures being formed in said wall means; and said windows being solocated in said apertures that the inner surfaces of the windows areflush with the inner surface of the wall means.
 12. The combination of atransducer for outputting a signal indicative of the concentration of aspecified gas in a sample which may contain that gas and an airwayadapter with means for confining said sample to a particular pathtraversing the transducer;said transducer comprising a housing, aninfrared radiation emitter, and an infrared radiation detector means;said emitter and said detector means being located in said housing; theairway adapter means for confining said sample to said particular pathbeing a flow passage through the airway adapter; and said airway adapterfurther comprising:means for supporting said transducer housing fromsaid airway adapter with said infrared radiation emitter and saidinfrared radiation detector means on opposite sides of the airwayadapter flow passage; apertures on opposite sides of the flow passagewhich are aligned along an optical path between said infrared radiationemitter and said detector means and thereby so allow infrared radiationto pass from said emitter through said airway adapter and the gas(es) inthe flow passage to said detector means that infrared radiation of thewavelengths absorbed by said specified gas is attenuated before itreaches said detector means so that the signal emitted by said detectormeans reflects the concentration of the specified gas in said sample;and windows fabricated of an infrared radiation transmitting polymer soinstalled in said apertures as to: (1) keep said gases from escapingthrough said apertures, and (2) transmit infrared radiation outputtedfrom said emitter to the flow passage and, after it traverses gas(es)therein, from the flow passage to said infrared radiation detectormeans; the perimeter of each said airway adapter aperture having: aninner segment configured to provide a retainer stop for locating thewindow in that aperture relative to a boundary of the airway adapterflow passage and an outer, undercut segment for holding said retaineragainst said stop and immobilizing said window in the selected locationrelative to the boundary of the airway adapter flow passage.